As discussed in a recent epic Perspective on the animal-microbe symbioses (and commented in this blog), we are beginning to understand the many contributions made by each partner. One of the most widely studied symbioses is that of the luminous bacterium Vibrio fischeri and the Hawaiian Bobtail squid, Euprymna scolopes. In a recent publication, Heath-Heckman and coworkers determined that bacterial bioluminescence regulates the expression of a squid circadian rhythm (cryptochrome) gene (cryptochromes are a family of blue light receptor proteins involved in circadian rhythms of animals and plants).